Trello is a free web-based project management application made by Fog Creek Software.

Trello uses a paradigm for managing projects known as kanban, a method that had originally been popularized by Toyota in the 1980s for supply chain management. Projects are represented by boards, which contain lists (corresponding to task lists). Lists contain cards (corresponding to tasks). Cards are supposed to progress from one list to the next (via drag-and-drop), for instance mirroring the flow of a feature from idea to implementation. Users can be assigned to cards. Users and boards can be grouped into organizations.

Trello boards and cards are barely-metaphors for their antecedants: notecards pinned to a cork-board.
This terminology left me uncomfortable for days, as I didn’t understand what it meant -- I only found it buried in Wikipedia, and explained at this post.
I’m still a little uncomfortable, as I don’t see how the continued use of this opaque metaphor will mean anything to Trello’s target market, NON-developers. They’ve stated (see Joel @ How Trello is different) that they want Trello to be a horizontal product, one that can be used by anybody.
Heck, I am a developer, and I’m not even familiar with the metaphor (not being in an Agile shop).